CBEST Test Prep
CBEST Math Test Tips - Page six.
Mental Math Tips
On multiple choice tests, you may not need an exact answer.
This method works for whatever percentage you are asking for.
Examples:
Find 27% of 568.
10%=56.8, 1% = 5.68, therefore, 27% = 30% - 3%, or 30% -1%-1%-1%.
Just do 56.8 + 56.8 + 56.8 - 5.68 - 5.68 - 5.68.
You may not have to do the whole problem to find the answer.
You can round and get a very close estimate: 57 +57 + 57 - 18 = 153. The answer MUST be very close to 153. Eliminate all answers that are not even close. The exact answer is 153.36.
Example: What number is 30% of 400 ?
Again, the rapid 10%, 1%, or 5% will help us.
10% = 40, so 30% = 40 + 40 + 40 = 120. Easy.
It will also help us to set boundaries, and look for answers
that are
close.
Example:
Find 48% of 798.
We should now have a working knowledge of the fact that 50% = half. So, 50% of 798 = 350 + 45 + 4, or 399.
We know the answer must be less than 399. We also know that 48% = 50% - 2%. So, we can do 399 - 7.98 - 7.98. Again, we can round to get 399 - 8 - 8, or about 383. The exact answer is 383.04. If we need an exact answer, we could go back and not round.
The other way percentage problems work, is you are given a
ratio or
fraction and you are asked to find the percent.
Examples:
46 is what percentage of 69?
First we should decide how 46 fits. Is it more or less than 50% ?
Obviously
it is more, since half of 69 is 34.5.
1% = .69, 10% = 6.9, 5% = 3.45. 50% + 10% = 34.5 + 6.9, or 41.4. Add
another 5% or 3.45, and we get 44.85. We have now added a total of 65%
( 50% + 10% + 5%). The answer is a little more than 65%. I could even
add
another 1% to get 45.54. Very close to 66%. In fact, the real answer is
roughly 66.66%
Suppose the problem is like the following:
A class has 32 boys and girls. 1/4th of the class has brown hair. What
is the total number of students that has brown hair? I know I need to
find
1/4 of 32 or 25%. Finding 1/4 is the same thing as dividing by 4. 32/4
= 8.
Make sure you are finding what they ask for. Suppose the
question is
as follows:
Out of 750 students, 3/5 are not taking P.E. classes. How many of the
total students are taking P.E. classes? Here the question is to
actually
find the other 2/5. But I can do it either way. If I find 3/5, I
subtract
that from the total, 750, to get my answer. Let's work this one out.
2/5 of 750 is less than 50%. I can eliminate answers that are more
than 50%, which is 350 + 25, or 375. I also know that 2/5 is equal to
40%.
Since I know 50%, I can do 50% - 10% to get 40%, or 375 - 75, or
300. That's my answer.
To do it the other way, 3/5 of 750 is equal to 60%, or 50% + 10%. This equals 375 + 75, or 450. But, I need to ask myself "What have I found?" I have found the total of those who ARE NOT taking P.E. I simply subtract 750 - 450 = 300 to get the number I need.
More CBEST Test Prep Resources
CBEST Test Tips - Overall tips to take and pass the CBEST Test.
CBEST Math Tips - Read our tips to pass the math section of the CBEST Test.
Essay Writing Tips for the CBEST - Get our CBEST essay writing guide here.
* Privacy Policy